A conventionally known shift control system of an automatic transmission is disclosed in Japanese Patent Unexamined Publication No. 2000-240776.
Described in the above publication includes a technology for changing gear from first speed 1ST, second speed 2ND and third speed 3RD to third speed 3RD and from fourth speed 4TH, fifth 5TH and sixth speed 6TH to sixth speed 6TH in a case of an electric failure caused to a traveling vehicle.
Thereby, change of a gear to lower than a gear of the traveling vehicle can be avoided, thus preventing sudden engine brake or engine's excessive high speed. In addition, the third speed 3RD capable of causing a great driving force sufficient for start signifies that the driving force can be secured for the vehicle to travel.
As is seen in FIG. 9 (FIG. 11 on page 20 of the above publication), the shift control system of the automatic transmission according to the above conventional technology, however, changes the gear from the first speed 1ST, second speed 2ND and third speed 3RD to the third speed 3RD while from the fourth speed 4TH, fifth 5TH and sixth speed 6TH to the sixth speed 6TH in the electric failure by means of a fail safe valve 77, a fail safe valve 78, a fail safe valve 79, a fail safe valve 80 and a fail safe valve 81 each of which operates with pressure of one of respective friction elements as an operation signal pressure. Therefore, the shift control system of the automatic transmission according to the above conventional technology may cause an insensible failure which may be attributable, for example, to the fail safe valve 77 sticking to a spring's extension side (hereinafter, the insensible failure is referred to as a sleeping failure), thus causing an interlock.
Namely, a duty solenoid SLC1 cutting a C-1 hydraulic pressure contributes to trouble-free (sleeping failure), when the fail safe valve 77 sticks (in failure) to the spring's extension side at an ordinary fifth speed 5TH. In this state, however, the following failures may cause the interlock attributable to simultaneous engagements of a plurality of friction elements: 1. the electric failure, 2. cutting of the duty solenoid SLC1, or 3. the duty solenoid SLC1 sticking to the hydraulic pressure output side. Substantially the like phenomenon is supposed to hold true to the fail safe valve 78, the fail safe valve 79, the fail safe valve 80 and the fail safe valve 81 in the sleeping failure.
Disposed between the duty solenoids and the friction elements include the fail safe valve 77, the fail safe valve 78, the fail safe valve 79, the fail safe valve 80 and the fail safe valve 81 and the like each of which operates with the pressure of one of the respective friction elements as the operation signal pressure. With the above constitution, changing the gear may output three or more of the hydraulic pressures substantially simultaneously. Thereby, controlling at high hydraulic pressure may cause malfunction to the fail safe valve 77, the fail safe valve 78, the fail safe valve 79, the fail safe valve 80 and the fail safe valve 81 and the like, thus restricting hydraulic pressure control range and deteriorating shift controllability.
In addition, achieving the gear change from the first speed 1ST, second speed 2ND and third speed 3RD to the third speed 3RD while from the fourth speed 4TH, the fifth speed 5TH and sixth speed 6TH to the sixth speed 6TH may use three long (overall length) fail safe valves (namely, the fail safe valve 77, the fail safe valve 78, and the fail safe valve 81) having two shifts, and two fail safe valves (namely, the fail safe valve 79, and the fail safe valve 80), thus complicating the layout and increasing a control valve unit in size.
In addition, the gear change from the fourth speed 4TH, the fifth speed 5TH and the sixth speed 6TH to the sixth speed 6TH in the electric failure involves a maximum gear change of two (fourth speed 4TH to sixth speed 6TH), thus causing an excessive shock (down spike and up spike).
In other words, the gear change in the electric failure from the fourth speed 4TH to the sixth speed 6TH is likely to cause the interlock due to an excessive capacity of both the engagement and disengagement as is seen in FIG. 10, thus causing the down spike. Moreover, a momentary great change in turbine speed may cause the up spike. En passant, FIG. 11 shows a resultant diagram of an up shift simulation of changing the gear from the fourth speed 4TH to the sixth speed 6TH in the case of fuse cut (electric failure) caused to the vehicle traveling at 230 km/h with fourth speed 4TH. The up shift simulation from the fourth speed 4TH to the sixth speed 6TH causes deceleration of −0.25 G and up spike acceleration of +0.21 G, thus causing acceleration-deceleration range of 0.46 G (>0.4 G).